In any modern IDE “dredging through old commits” means clicking a single button to see who last changed the line. From there it often makes sense to go look at the PR to see a higher level of what was changed. You cannot include all of that context in a single comment.
Valve makes infinitely more money off of store fees selling other people’s games. Why would they want to make a fraction developing a single game when they can go after Android’s 30% cut?